The present invention relates to an apparatus for coupling a signal traveling in a waveguide onto a microstrip. More specifically, the invention relates to the construction of a waveguide to microstrip transition as a directly fabricated and hermetically sealed packaging structure, which may also connect the microstrip to integrated circuits operating at millimeter-wave or microwave frequencies.
Both waveguides and microstrips are devices that transport electromagnetic energy (hereafter "signals") from point to point. A waveguide is generally implemented as a conduit with an inner electromagnetically reflecting surface. Signals introduced into the interior of the waveguide propagate along the interior of the waveguide by reflecting back and forth between the walls of the waveguide. Waveguides are generally circular or rectangular in cross-section and provide for low loss transmission of the signals.
Waveguides have a critical wavelength for passage of signals within, related to the waveguide geometry. Signals with wavelengths greater than the critical wavelength are unable to propagate in the waveguide. Thus, the structure of the waveguide intrinsically serves to filter out undesired signals. It is often necessary, for example in satellite applications, to process the signal with integrated circuitry. However, a complication involved in constructing waveguides connected to integrated circuitry is that the waveguide structure tends to be large in relation to the integrated circuitry. Thus, in the past, waveguides required additional fabrication steps, incurred additional cost, and suffered from lower reliability when used in conjunction with integrated circuitry, including microstrips.
A microstrip is a thin conducting strip placed on top of a dielectric material. The dielectric material, in turn, is supported by a conducting plate, typically grounded. A microstrip can be fabricated directly in an integrated circuit process by depositing a thin conducting strip on the surface of the substrate. An entire set of integrated circuit electronics, including a microstrip, often occupies far less geometric volume than the corresponding waveguide structure. Microstrips are suitable for use as a high bandwidth, miniaturized, cost effective signal transmission line. Because of these advantages, microstrip lines are used in a great number of commercial microwave and millimeter-wave applications.
Many commercial and military satellite systems, for example, use microstrips coupled to waveguides in different portions of the system. For example, a satellite antenna may guide an uplink beam through a waveguide, then to a downconverter connected to a microstrip for processing. Thus, a key factor in the operation of commercial and military satellite systems is a waveguide to microstrip transition which provides low loss signal coupling between the microstrip and the waveguide.
One patent related to coupling signals between waveguides and microstrips is U.S. Pat. No. 4,453,142 to Earl R. Murphy entitled Microstrip to Waveguide Transition. Murphy discloses a microstrip to waveguide transition for use in the millimeter-wave frequency range. Murphy's waveguide consists of a solid metal rectangular waveguide that is attached to a substrate. A tab of the substrate carrying the microstrip extends into the waveguide.
The solid metal waveguide used in Murphy is rigidly connected to the base, for example by bolting. A wall of the waveguide is pierced to provide an opening for the microstrip. The base, waveguide, and microstrip are assembled to provide a waveguide to microstrip transition. Because the microstrip extends through a metallic wall of the waveguide, the microstrip is necked to help minimize shunt capacitance between the microstrip and the waveguide. Murphy, however, does not address the difficulties associated with forming a waveguide to microstrip transition in a single, easy to manufacture integrated circuit package.
Other prior waveguide to microstrip designs use microstrip, stripline, and coaxial interfaces to construct input/output ports to the microstrip. While these interfaces serve to provide low loss transmission of the internal signal to the external connections over a wide range of frequencies, acceptable performance of these interfaces requires complicated and time consuming circuit tuning unsuitable for large volume manufacturing. Furthermore, previous approaches employing multi-layer Low Temperature Co-fired Ceramic (LTCC) substrates for monolithic millimeter-wave integrated circuit (MMIC) packaging applications have been limited by the input/output frequency (microwaves) interface. The conventional interface uses microstrip and stripline structures to launch microwaves onto the substrate. The conventional interface yields poor electrical performance at millimeter-wave frequencies because of parasitic inductance and capacitance and other high frequency effects, thereby limiting usefulness to frequencies below 40 GHz.
A need remains for an improved transition structure from waveguide to microstrip on multi-layer substrate which overcomes the disadvantages discussed above and previously experienced.